Lithium secondary battery

ABSTRACT

Provided is a lithium secondary battery including a cathode containing a cathode active material in which a central part has a different concentration from a surface part, and a conductive material having a specific composition ratio, and specifically, a lithium secondary battery including a cathode containing a cathode active material in which a central part of one or more kinds of metals configuring the cathode active material has a different concentration from a surface part thereof, and two or more kinds of conductive materials mixed at a specific ratio, thereby having excellent stability and high low-temperature characteristic and high output characteristic as compared to a conventional lithium secondary battery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/371,936 filed on Dec. 7, 2016, which claims priority under 35 U.S.C.§119 to Korean Patent Application No. 10-2015-0175166, filed on Dec. 9,2015, in the Korean Intellectual Property Office, the disclosure ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The following disclosure relates to a lithium secondary battery, andmore specifically, to a lithium secondary battery wherein a cathodeincludes a cathode active material in which a central part of one ormore kinds of metals configuring the cathode active material has adifferent concentration from a surface part thereof, and two or moreconductive materials mixed at a specific ratio, thereby having excellentstability and high low-temperature characteristic and high outputcharacteristic.

BACKGROUND

In accordance with technology development and an increase in demand formobile devices, the demand for secondary batteries as energy sources hasbeen rapidly increasing. Among these secondary batteries, a lithiumsecondary battery having high energy density and voltage has beencommercialized and widely used.

In order to use the lithium secondary battery as a power source for anelectric vehicle, a hybrid electric vehicle, etc., the lithium secondarybattery is required to have performance in which it is capable ofoperating under harsher conditions in a mobile phone, a notebook, apersonal digital assistant (PDA), etc. Since vehicles need to operate ata low temperature, for example, during the winter, a representativeexample of such a requirement is excellent output characteristic at alow temperature.

Meanwhile, a cathode active material among components of the lithiumsecondary battery plays an important role in determining performancessuch as battery capacity, an output characteristic, etc., in thebattery.

Lithium cobalt oxide (LiCoO₂), which has relatively excellent generalphysical properties such as excellent cycle characteristic, etc., ismainly used as a cathode active material. However, cobalt used inLiCoO₂, which is a rare metal, has a small amount of reserves andproduction are unevenly distributed, and thus, supply is unstable. Inaddition, LiCoO₂ has a problem of high cost due to instability of cobaltsupply and an increase in demand of the lithium secondary battery.

Accordingly, a study on the cathode active material capable of replacingLiCoO₂ has been conducted steadily, and in particular, a technique ofreplacing cobalt with manganese, nickel, etc., has been considered, butit is difficult to apply this replacing technique to actual massproduction or the cycle characteristic is reduced.

In order to overcome these disadvantages, a method of using a lithiumcomposite transition metal oxide or a lithium transition metal phosphatecontaining two or more kinds of transition metals as the cathode activematerial has been studied. However, there are still problems such asreduction in capacity, reduction in high-rate characteristic, etc., dueto instability of the transition metal. In particular, a compositetransition metal oxide containing iron has higher electrical resistancein the same state with the same open circuit voltage (OCV), and thus,the composite transition metal cathode active material still has aproblem of insufficient output characteristic despite advantages of lowcost and high safety. Further, a problem of reduction in capacity andoutput of the battery due to a decrease in electrochemical reaction rateat a low temperature is serious.

Therefore, there is a high need for a lithium secondary batterytechnology having a high capacity under low-temperature condition andexcellent output characteristic.

RELATED ART DOCUMENT

(Patent Document 1) Korean Patent Laid-Open Publication No.10-2012-0130715 (Dec. 3, 2012)

SUMMARY

An embodiment of the present disclosure is directed to providing alithium secondary battery including a cathode containing a cathodeactive material in which a central part of one or more kinds of metalsconfiguring the cathode active material has a different concentrationfrom a surface part thereof, and two or more kinds of conductivematerials mixed at a specific ratio, thereby having excellent stabilityand high low-temperature characteristic and high output characteristic.

The present disclosure relates to a lithium secondary battery.

In one general aspect, a lithium secondary battery includes a cathode,an anode, an electrolyte, and a separator, wherein the cathode includesa cathode active material in which a transition metal concentration of acentral part is different from that of a surface part, and a conductivematerial in which a carbon large surface area structure and a carbonsmall surface area structure are mixed at a weight ratio of 10:90 to70:30.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a cathode active material according toan exemplary embodiment of the present disclosure.

FIG. 2 is a scanning electron microscope (SEM) image showing a crosssection of a cathode active material prepared by Examples 1 to 6 of thepresent disclosure.

FIG. 3 is a scanning electron microscope (SEM) image showing a crosssection of a cathode active material prepared by Examples 7 to 12 of thepresent disclosure.

FIG. 4 is a scanning electron microscope (SEM) image showing a crosssection of a cathode active material prepared by Comparative Examples 1to 6 of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, a lithium secondary battery according to the presentdisclosure is described in more detail with reference to specificexemplary embodiments of the present disclosure. Meanwhile, specificexemplary embodiments and Examples are provided as a reference forexplaining the present disclosure in detail, and therefore, the presentdisclosure is not limited thereto, but may be implemented in variousways.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings generally understood by those skilled in the artto which the present disclosure pertains. Terms used in thespecification of the present disclosure are merely provided toeffectively describe specific exemplary embodiments, but are notintended to limit the present disclosure.

The present inventors continuously studied to overcome disadvantages ofa conventional lithium secondary battery, i.e., a low discharge outputand a reduction in discharge capacity at a low temperature, and foundthat when a cathode active material in which a concentration of acentral part is different from that of a surface part was used, andsimultaneously, a composition ratio of two or more conductive materialsincluded in a cathode was controlled, it was able to improve a dischargeoutput, and to suppress a reduction in discharge capacity at a lowtemperature, and as a result, completed the present disclosure.

A lithium secondary battery according to the present disclosure includesa cathode active material in which a central part of one or more metalshas a different concentration from a surface part thereof and aconductive material having a specific composition, such that a dischargeoutput is 3,000 W/kg, and more preferably 3,300 W/kg or more, and adischarge capacity at −20° C. as compared to room temperature (25° C.)is 75% or more, and more preferably, 80% or more.

The lithium secondary battery according to the present disclosure mayinclude a cathode, an anode, an electrolyte, and a separator, whereinthe cathode may include a cathode active material in which a centralpart of one or more metals has a different concentration from a surfacepart thereof; and a conductive material in which a carbon large surfacearea structure and a carbon small surface area structure are mixed at aweight ratio of 10:90 to 70:30.

A structure of the cathode active material prepared through the presentdisclosure will be described in more detail with reference to FIG. 1showing a cross section of the cathode active material. The cathodeactive material according to the present disclosure may be divided intofrom the central part 1 to the surface part 13 on the basis of the crosssection. It is noted that numbers shown in FIG. 1 are arbitrarilydescribed to divide the central part and the surface part, and thepresent disclosure is not limited thereto. In addition, the central partor the surface part is not limited to only a region of the singlenumber. For example, the central part may occupy 1 to 5 regions in FIG.1, and the surface part may have the other regions. This also applies toa case where the central part is divided into a first central part and asecond central part.

Hereinafter, the cathode active material according to the presentdisclosure is described in more detail through a preparation method.

The preparation method of the cathode active material according to thepresent disclosure specifically includes a) forming a central part bysimultaneously mixing a lithium source material, one or more transitionmetal source materials, a chelating agent, and a basic aqueous solution,followed by firing; b) preparing a compound for forming a surface partby simultaneously mixing the lithium source material, one or more of thetransition metal source materials, the chelating agent, and the basicaqueous solution, followed by firing, and grinding into a nano-size; c)forming the surface part on a surface of the central part by mixing thecentral part obtained in step a) and the compound for forming a surfacepart obtained in step b); and d) forming a structure in which a sectionhaving a different transition metal concentration exists between thecentral part and the surface part by heat treating the compound obtainedin step c).

Further, in the present disclosure, when the structure of the cathodeactive material includes the first central part, the second centralpart, and the surface part, the preparation method of the cathode activematerial may include a) forming a first central part by simultaneouslymixing a lithium source material, one or more transition metal sourcematerials, a chelating agent, and a basic aqueous solution, followed byfiring; b) preparing a compound for forming a second central part bysimultaneously mixing the lithium source material, one or more of thetransition metal source materials, the chelating agent, and the basicaqueous solution, followed by firing, and grinding into a nano-size; c)preparing a compound for forming a surface part by simultaneously mixingthe lithium source material, one or more of the transition metal sourcematerials, the chelating agent, and the basic aqueous solution, followedby firing, and grinding into a nano-size; d) forming the second centralpart on a surface of the first central part by mixing the first centralpart obtained in step a) and the compound for forming a second centralpart obtained in step b); e) forming the surface part on a surface ofthe second central part by mixing the compound obtained in step d) andthe compound for forming a surface part obtained in step c); and f)forming a structure in which a section having a different transitionmetal concentration exists among the first central part, the secondcentral part, and the surface part by heat treating the compoundobtained in step e).

When the cathode active material according to the present disclosure hasthe structure of the central part and the surface part, firstly, in stepa), the central part may be formed by simultaneously mixing the lithiumsource material, one or more of the transition metal source materials,the chelating agent, and the basic aqueous solution, followed by firing.

The lithium source material in the present disclosure is not limited inview of a kind as long as it is a material generally used in preparationof the cathode active material, etc., in the art, and for example, thelithium source material is not particularly limited as long as it is alithium salt such as lithium carbonate, lithium nitrate, etc.

In the present disclosure, the transition metal source material mayinclude a metal salt of at least one element selected from the groupconsisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe),sodium (Na), calcium (Ca), titanium (Ti), vanadium (V), chromium (Cr),copper (Cu), zinc (Zn), germanium (Ge), strontium (Sr), silver (Ag),barium (Ba), zirconium (Zr), niobium (Nb), molybdenum (Mo), aluminum(Al), gallium (Ga), boron (B), and a combination thereof. In addition,the metal salt may be a sulfate, a nitrate, an acetate, a halide, ahydroxide, etc., and is not particularly limited as long as it iscapable of being dissolved in a solvent.

More specifically, as the transition metal source material in thepresent disclosure, M1 may be a nickel (Ni) salt, M2 may be a cobalt(Co) salt, and M3 may be a manganese (Mn) salt in Chemical Formulas 1 to5. In addition, the transition metal source material may be mixed bycontrolling molar ratio so as to have high capacity characteristic. Themolar ratio may be easily controlled according to metal composition ofthe central part to be obtained, and the molar ratio of the transitionmetal may be, for example, X1+Y1+Z1=1 in Chemical Formula 1.

The chelating agent used in the present disclosure may be an aqueousammonia solution, an aqueous ammonium sulfate solution, or a mixturethereof, and the molar ratio between the chelating agent and thetransition metal source material may be 0.1 to 0.5:1, but the presentdisclosure is not limited thereto.

Examples of the basic aqueous solution used in the present disclosuremay include sodium hydroxide, potassium hydroxide, etc., but the basicaqueous solution is not limited thereto, and may be any materialregardless of the type as long as it is a basic material that isgenerally usable in preparation of the active material. In addition, thebasic aqueous solution may have a concentration of 1 to 5 M, but thepresent disclosure is not limited thereto.

In the present disclosure, a co-precipitation method may be applied instep a). More specifically, one or more transition metal salts aredissolved in a solvent such as distilled water, etc., and then, achelating agent and a basic aqueous solution are continuously introducedinto a reactor, respectively, to generate precipitation. Here, anaverage residence time of a transition metal salt solution in thereactor may be controlled to 2 to 12 hours, pH may be controlled to 10to 12.5, preferably 10.5 to 11.5, and a temperature of the reactor maybe controlled to 50 to 100° C. Further, a reaction time in the reactormay be controlled to 5 to 40 hours, preferably 10 to 30 hours. However,these conditions may be freely changed depending on a composition of thesource materials, a composition ratio thereof, etc., and the presentdisclosure is limited thereto.

A precipitate prepared through the reactor may be collected in a slurryform, and then, this slurry solution may be filtered, washed and driedto obtain a metal composite oxide. The metal composite oxide may bemixed with a lithium source material at a predetermined ratio, followedby heat firing at 900 to 1,000° C. under air flow, thereby forming thecentral part. A ratio of the lithium source material and the metalcomposite oxide is not limited, but is preferably a weight ratio of 1:1.

Next, in step b), the compound for forming a surface part may beprepared by simultaneously mixing the lithium source material, one ormore of the transition metal source materials, the chelating agent, andthe basic aqueous solution, followed by firing, and grinding into anano-size.

The transition metal source material formed on the surface part in thepresent disclosure may be the same as or different from the transitionmetal source material used in the formation of the central part. Morespecifically, similar to the formation of the central part, as thetransition metal source material of the surface part, M1 may be a nickel(Ni) salt, M2 may be a cobalt (Co) salt, and M3 may be a manganese (Mn)salt. Further, the transition metal source material may be mixed bycontrolling molar ratio so as to have high capacity characteristic. Themolar ratio may be easily controlled according to metal composition ofthe central part to be obtained, and the molar ratio of the transitionmetal may be, for example, X1+Y1+Z1=1 in Chemical Formula 1.

The kind and amount of the chelating agent and the basic aqueoussolution used for preparing the compound for forming a surface part inthe present disclosure may be the same as or different from those of thecentral part, and the present disclosure is not limited thereto.

In the present disclosure, step b) may be performed by theco-precipitation method which is the same as step a). Here, an averageresidence time, pH, and a reaction time, etc., of the transition metalsalt solution in step b) may be the same as or different from step a),but the present disclosure is not limited thereto. In addition, dryingof the precipitate obtained through the reactor and the mixing of thelithium source material may also be performed under the same conditionsas step a). The ratio of the lithium source material and the metalcomposite oxide (precipitate) is not limited, but is preferably 1:1.

The compound for forming the surface part obtained in step b) may beground into several nanometers using an air jet mill. Accordingly, it ispossible to improve electrical conductivity of the cathode activematerial to be prepared.

Next, in step c), the surface part may be formed on the surface of thecentral part by mixing the central part obtained in step a) and thecompound for forming a surface part obtained in step b). In this step, amethod of forming the surface part is not limited. For example, thecompound for forming a central part and a surface part may be put into ahigh-speed dry coater and mixed at a speed of 1,000 to 50,000 rpm.Accordingly, the compound for forming a surface part may be coated onthe surface of the central part while being surrounded with apredetermined thickness. Further, in step c), a thickness of the surfacepart coating the central part may be controlled by adjusting a retentiontime, a temperature and a rotation speed in the coater.

The obtained compound may be subjected to a heat treatment as step d) toform a structure in which the transition metal has a differentconcentration between the central part and the surface part. Here, atemperature for heat treatment is not limited in the present disclosure,but may be 300 to 1,000° C., and an atmosphere may also be an oxidizingatmosphere such as air, oxygen, or the like. Further, time for heattreatment may be 10 to 30 hours, and a pre-firing process may beperformed by maintaining the temperature at 150 to 800° C. for 5 to 20hours before the heat treatment, or an annealing process may beperformed at 600 to 800° C. for 10 to 20 hours after the heat treatment.

In the present disclosure, when the structure of the cathode activematerial includes the first central part, the second central part, andthe surface part, the cathode active material may be prepared in thesame manner as the cathode active material having a two-layer structureof the central part and the surface part. Specifically, when the secondcentral part is coated on the first central part, a composition forforming a second central part may be firstly ground, and put into areactor, stirred and coated, and then, the surface part may be formed onthe surface of the second central part in the same manner as above.

The prepared cathode active material may include a conductive sourcematerial, followed by milling, thereby preparing a slurry for preparinga cathode including the conductive material and the cathode activematerial.

The conductive source material usable in the present disclosure may bedivided into a carbon large surface area structure and a carbon smallsurface area structure on the basis of a shape.

The term “carbon large surface area structure” used herein means amaterial in which one particle consisting of carbon atoms arranged in ahexagonal shape has a surface area of 5,000 nm² or more, and may includeall carbon structures that mainly do not have a point shape such as atwo-dimensional surface or a three-dimensional cylinder.

Further, the term “carbon small surface area structure” used hereinmeans a material in which one particle consisting of carbon atomsarranged in a hexagonal shape has a surface area of less than 5,000 nm²,and may include all carbon structures having a point shape rather thanthe carbon large surface area structure.

An example of the carbon large surface area structure in the presentdisclosure may be any one or more selected from graphene, single-walledcarbon nanotube, and multi-walled carbon nanotube.

An example of the carbon small surface area structure in the presentdisclosure may be any one or more selected from fullerene; black leadsuch as graphite, natural black lead, artificial black lead, etc.;carbon black, acetylene black, KETJENBLACK™, DENKA BLACK™, thermalblack, channel black, furnace black, lamp black, thermal black, andSUPER P™.

As the conductive source material in the present disclosure, conductivefibers such as carbon fiber, metal fiber, etc.; fluorinated carbon;metal powders such as aluminum powder, nickel powder, etc., conductivewhiskers such as zinc oxide and potassium titanate, etc.; conductiveoxides such as titanium oxide, etc.; and conductive polymers such aspolyphenylene derivative, etc., may be used alone or two or more thereofmay be mixed in addition to the carbon large surface area structure andthe carbon small surface area structure, but the present disclosure isnot necessarily limited thereto.

The conductive source material used in the present disclosure may bemore specifically a mixture of carbon nanotube and carbon black. It isintended to induce formation of a three-dimensional network structure bymixing conductive materials having different structural characteristicsso that point, line and surface contact are formed together between theconductive materials. Specifically, the carbon nanotube has acylindrical shape and has a large specific surface area. Meanwhile, thecarbon black has a nano-sized spherical shape, and thus, the carbonblack may be easily adsorbed on the surface of the carbon nanotube toform the three-dimensional network structure. When the three-dimensionalnetwork structure is formed, π-π interaction between the conductivesource materials may be reduced, and as a result, it is possible torather suppress reduction in the electrical characteristics caused byre-aggregation of the respective conductive source materials.

In the present disclosure, with regard to a mixing ratio of theconductive source material, the carbon large surface area structure andthe carbon small surface area structure may be mixed at a weight ratioof 10:90 to 70:30, and more specifically, a weight ratio of 20:80 to40:60. When the carbon large surface area structure is added at a weightratio of less than 10 or the carbon small surface area structure isadded at a weight ratio of more than 90, contactability between theconductive material and the cathode active material is reduced, whichcauses a decrease in the discharge output and the low-temperaturecharacteristic. When the carbon large surface area structure is added ata weight ratio of more than 70 or the carbon small surface areastructure is added at a weight ratio of less than 30, it is notpreferable since the contactability between the conductive material andthe cathode active material is reduced.

The cathode active material according to the present disclosure may havea different concentration between the central part and the surface part,and may include the central part represented by Chemical Formula 1 andthe surface part represented by Chemical Formula 2 in the order from aninside to an outside:

LiM1_(X1)M2_(Y1)M3_(Z1)O_(w)  [Chemical Formula 1]

LiM1_(X2)M2_(Y2)M3_(Z2)O_(w)  [Chemical Formula 2]

in Chemical Formulas 1 and 2, M1, M2, and M3 are selected from the groupconsisting of Ni, Co, Mn, Fe, Na, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba,Zr, Nb, Mo, Al, Ga, B, and a combination thereof, 0.75≤X1≤0.9,0.75≤X2≤0.9, 0≤Y1≤0.2, 0≤Y2≤0.2, 0.01 Z1≤0.2, 0.01≤Z2≤0.2, 0≤W≤3,0<X1+Y1+Z1≤1, 0<X2+Y2+Z2≤1, X2≤X1, Y1≤Y2, and Z1≤Z2, except that theChemical Formula 1 and the Chemical Formula 2 are the same.

In addition, in another embodiment of the cathode active material for alithium secondary battery according to the present disclosure, theconcentration of the metal may be different from the central part to thesurface part, and the cathode active material may include a firstcentral part represented by Chemical Formula 3 below, a second centralpart represented by Chemical Formula 4 below, and the surface partrepresented by Chemical Formula 5 below in the order from the inside tothe outside:

LiM1_(X3)M2_(Y3)M3_(Z3)O_(w)  [Chemical Formula 3]

LiM1_(X4)M2_(Y4)M3_(Z4)O_(w)  [Chemical Formula 4]

LiM1_(X5)M2_(Y5)M3_(Z5)O_(w)  [Chemical Formula 5]

in Chemical Formulas 3 to 5, M1, M2, and M3 are selected from the groupconsisting of Ni, Co, Mn, Fe, Na, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba,Zr, Nb, Mo, Al, Ga, B, and a combination thereof, 0.75≤X3≤0.9,0.75≤X4≤0.9, 0.75≤X5≤0.9, 0≤Y3≤0.2, 0≤Y4≤0.2, 0≤Y5≤0.2, 0.01≤Z3≤0.2,0.01≤Z4≤0.2, 0.01≤Z5≤0.2, 0≤W≤3, 0<X3+Y3+Z3≤1, 0<X4+Y4+Z4≤1,0<X5+Y5+Z5≤1, X5≤X4≤X3, Y3≤Y4≤Y5, Z3≤Z4≤Z5, except that the ChemicalFormulas 3 to 5 are the same, respectively, or all of the ChemicalFormulas 3 to 5 are the same.

More specifically, in Chemical Formulas 1 to 5 of the presentdisclosure, M1 may be Ni, M2 may be Co, and M3 may be Mn. By usingnickel, cobalt, and manganese as the transition metals for the cathodeactive material, and simultaneously, by controlling the compositionratio as shown in Chemical Formulas 1 to 5, it is possible to suppressoverdischarge of a lithium secondary battery to be produced, andsimultaneously, to suppress occurrence of impurities such as lithiumhydroxide (LiOH), lithium carbonate (Li₂CO₃), etc., and as a result, thecapacity of the battery may be increased.

In the present disclosure, it is possible to have a continuous gradientof the concentration of the transition metal included in the cathodeactive material from the central part to the surface part by controllingthe molar ratio of M1 to M3 in Chemical Formulas 1 to 5.

In particular, in Chemical Formulas 1 to 5 of the present disclosure,when M1 is Ni, M2 is Co, and M3 is Mn, the molar ratio of M1 may bedecreased from the central part to the surface part, and the molar ratioof M3 may be increased from the central part to the surface part.Specifically, the molar ratio of M2, cobalt, may be fixed, and thecontent of nickel may be gradually reduced from the central part, andsimultaneously, the content of manganese may be gradually increased fromthe central part, and a molar ratio of the entire transition metal maybe fixed to a predetermined range.

More specifically, the cathode active materials represented by ChemicalFormulas 1 and 2 may satisfy Equations 1 and 2:

$\begin{matrix}{0.01 \leq {{{X\; 1} - {X\; 2}}} \leq 0.05} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack \\{0.01 \leq {{{Z\; 1} - {Z\; 2}}} \leq {0.05.}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

Further, the cathode active materials represented by Chemical Formulas 3to 5 may satisfy Equations 3 to 8:

$\begin{matrix}{0.01 \leq {{{X\; 3} - {X\; 4}}} \leq 0.05} & \lbrack {{Equation}\mspace{14mu} 3} \rbrack \\{0.01 \leq {{{X\; 4} - {X\; 5}}} \leq 0.05} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack \\{0.01 \leq {{{X\; 3} - {X\; 5}}} \leq 0.1} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack \\{0.01 \leq {{{Z\; 3} - {Z\; 4}}} \leq 0.05} & \lbrack {{Equation}\mspace{14mu} 6} \rbrack \\{0.01 \leq {{{Z\; 4} - {Z\; 5}}} \leq 0.05} & \lbrack {{Equation}\mspace{14mu} 7} \rbrack \\{0.01 \leq {{{Z\; 3} - {Z\; 5}}} \leq {0.1.}} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

Equations 1 to 8 show the difference in molar ratio of the transitionmetal between the central part and the surface part. When the cathodeactive material satisfies Equations 1 to 8, it is possible to suppressthe occurrence of impurities that may be caused due to a rapiddifference in composition of the transition metal, and to improve adischarge capacity and low-temperature characteristic of the lithiumsecondary battery to be produced.

More specifically, Equations 1 to 8 show the difference in molar ratioof the transition metal between the central part and the surface part,and particularly, when M1 is lithium, Equations 1 to 8 show a change inmolar ratio of lithium between the surface part and the central part,and a change in molar ratio of other transition metals according to thechange in molar ratio of lithium.

In general, since a concentration of lithium with regard to aconcentration of other transition metals is increased from the surfacepart to the central part, the change in concentration from the surfacepart to the central part needs to be clearly reflected, andsimultaneously, a rapid change in concentration needs to be prevented.

Equations 1 to 8 show the difference in molar ratio of the transitionmetal. When the difference in molar ratio of the transition metalbetween the central part and the surface part is less than the range ofEquations 1, 3 to 5, or more than the range of Equations 2, 6 to 8, thedifference in transition metal concentration between the central partand the surface part or between the first central part and the secondcentral part may not occur, and thus, excess transition metal carbonateor hydroxide remains on the surface, and a large amount of gas isgenerated at a high temperature at the time of cell assembly, such thata battery case is easy to be swollen, and at the time of mixing anelectrode for the cell assembly, gelation may easily occur, and when theelectrodes are coated, agglomeration may occur easily, which may causesurface defects.

On the contrary, when the difference in molar ratio of the transitionmetal between the central part and the surface part is more than therange of Equations 1, 3 to 5 or less than the range of Equations 2, 6 to8, a lithium concentration in the central part and the surface part maybe rapidly changed, and thus, the cathode active material may bestructurally unstable, which may cause defects of the secondary battery.

Further, in the cathode active material according to the presentdisclosure, thicknesses of the second central part and the surface partmay be controlled by controlling the residence time, the temperature,and the rotation speed in the coater at the time of forming the surfacepart or the second central part and the surface part. More specifically,the cathode active material represented by

Chemical Formulas 1 and 2 may satisfy Equation 9 below, and the cathodeactive material represented by Chemical Formulas 3 to 5 may satisfyEquations 10 and 11 below:

$\begin{matrix}{0.5 \leq {L_{1}/L} \leq {{0.9}9}} & \lbrack {{Equation}\mspace{14mu} 9} \rbrack\end{matrix}$

in Equation 9, L₁ is a distance from the center of the cathode activematerial to a boundary between the surface part and the central part,and L is a radius of an entire cathode active material.

$\begin{matrix}{0.05 \leq {L_{2}/L} \leq 0.5} & \lbrack {{Equation}\mspace{14mu} 10} \rbrack \\{0.01 \leq {L_{3}/L} \leq 0.2} & \lbrack {{Equation}\mspace{14mu} 11} \rbrack\end{matrix}$

in Equation 10, L₂ is a distance from the center of the cathode activematerial to a boundary between the first central part and the secondcentral part, L₃ is a distance from the boundary between the firstcentral part and the second central part to a boundary between thesecond central part and the surface part, and L is a radius of an entirecathode active material.

In the present disclosure, Equations 9 to 11 indicate a difference inconcentration of the transition metal of the cathode active material,which show a thickness of the central part in the entire cathode activematerial as a ratio. Specifically, the concentration gradient of thetransition metal from the central part to the surface part may becontrolled though the thicknesses of the central part and the surfacepart. When the concentration of the transition metal is excessively highon the surface part, the occurrence of impurities that may be caused maybe suppressed, and a rapid change in concentration of the central partand the surface part may be suppressed to increase structural stabilityof the cathode active material.

When the cathode active material has the range more than or less thanthat of Equations 9 to 11 in the present disclosure, the concentrationof a specific transition metal, particularly, lithium, may be increasedon the surface part, which may cause impurities such as carbonates,hydroxides, etc., of the transition metal, such that defects of thesecondary battery may occur.

The cathode active material according to the present disclosure has asection where the transition metal has a difference in concentrationfrom the central part to the surface part, thereby maintaining highcapacity, high energy density, and heat stability and simultaneouslyimproving a discharge capacity at a low temperature, such that thecathode active material may have excellent characteristics.

The cathode according to the present disclosure may be produced bypreparing a cathode active material composition including the cathodeactive material, the conductive material, a binder and a solvent, andthen, directly coating and drying the composition on a currentcollector. Otherwise, the cathode may be produced by casting the cathodeactive material composition on a separate support, and laminating a filmobtained by peeling from the support on an aluminum current collector.

A lithium secondary battery according to the present disclosure may havea structure in which an electrode laminate is impregnated with anelectrolyte, the electrode laminate including a plurality of stackedcathodes and anodes facing each other with separators interposedtherebetween.

Specifically, the lithium secondary battery according to an exemplaryembodiment of the present disclosure may include an electrode assemblyin which cathodes and anodes facing each other are alternately stackedwith separators interposed therebetween, an electrolyte impregnated inthe electrode assembly, and a battery case sealing the electrodeassembly and the electrolyte.

The respective cathodes of the electrode assembly may be connected inseries or in parallel or in serial-parallel, and the respective anodesmay also be connected in series or in parallel or in serial-parallel.Here, the cathode may include a current collector and a cathode activematerial layer containing a cathode active material on the currentcollector, and the cathode may include a non-coated part in which thecathode active material layer is not formed on the current collector.The anode may also include a current collector and an anode activematerial layer containing an anode active material on the currentcollector, and the anode may include a non-coated part in which theanode active material layer is not formed on the current collector.Electrical connection between the respective cathodes or between therespective anodes of the electrode assembly may be achieved through thenon-coated parts.

The current collectors of each cathode and/or each anode in theelectrode assembly may be porous conductors, and more specifically, thecurrent collector may be a foam, a film, a mesh, a felt, or a perforatedfilm made of conductive materials. More specifically, the currentcollector may be a conductive material having excellent conductivity andincluding graphite, graphene, titanium, copper, platinum, aluminum,nickel, silver, gold, or carbon nanotube that are chemically stable atthe time of charging and discharging the battery, and may have a shapeof a foam, a film, a mesh, a felt, or a perforated film made ofconductive materials, and may be a complex coated or stacked withdifferent conductive materials.

The anode active material of each anode in the electrode assembly isusable as long as it is an active material generally used in the anodeof the secondary battery. As one example of the lithium secondarybattery, the anode active material is used as long as it is capable ofperforming lithium intercalation. As one of the non-limiting examples,the anode active material may be one or more selected from anode activematerial groups of lithium (metal lithium), graphitizable carbon,non-graphitizable carbon, graphite, silicon, Sn alloy, Si alloy, Snoxide, Si oxide, Ti oxide, Ni oxide, Fe oxide (FeO), andlithium-titanium oxide (LiTiO₂, Li₄Ti₅O₁₂), and may be a composite of atleast two or more (a first anode active material and a second anodeactive material) selected from the anode active material groups. Thecomposite may have a structure in which the first anode active materialand the second anode active material are simply mixed, a core shellstructure including a core of the first anode active material and ashell of the second anode active material, a structure in which thesecond anode active material is supported on a matrix of the first anodeactive material, a structure in which the second anode active materialis coated or supported on the first anode active material having azero-dimensional nanostructure or one-dimensional nanostructure ortwo-dimensional nanostructure, or a structure in which the first anodeactive material and the second anode active material are stacked.

In the electrode assembly, the separators positioned between thecathodes and the anodes adjacent to each other may be different fromeach other or may be the same as each other per position according to astacked direction of the cathodes and the anodes, and are usable as longas they are separators generally used to prevent short-circuit of theanodes and the cathodes in general secondary batteries. As anon-limiting example based on the lithium secondary battery, theseparator may be one or more selected from polyethylene-based materials,polypropylene-based materials, polyolefin-based materials, andpolyester-based materials, and may have a microporous film structure.Here, the microporous film may be coated with an inorganic material. Inaddition, the separator may have a stacked structure in which aplurality of organic films such as polyethylene films, polypropylenefilms, and nonwoven fabric, and the like, are stacked to preventover-current, maintain the electrolyte, and improve physical strength.

The above-described electrode assembly may be produced by a productionmethod of a general jelly-roll type electrode assembly, and as oneexample, the electrode assembly may be formed by rolling a plurality ofcathodes and anodes that are spaced apart from each other while beingdisposed alternately to each other, on one surface of the separator.However, the present disclosure is not limited to the above-describedproduction method of the electrode assembly.

The electrolyte impregnated in the electrode assembly may be a generalaqueous or non-aqueous electrolyte capable of smoothly conductinglithium ions involved in charging and discharging of the battery andstably maintaining battery characteristics for a long period of time inconventional lithium secondary batteries.

An example of the electrolyte in the present disclosure may be anon-aqueous electrolyte, and the non-aqueous electrolyte may include anon-aqueous solvent and a lithium salt. One of the non-limiting examplesof the lithium salt contained in the electrolyte may be a salt providinga lithium cation and one or more anions selected from the groupconsisting of NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻,(CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻,(CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂ (CF₃)₂C_(O) ⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻,(CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and(CF₃CF₂SO₂)₂N⁻.

The solvent of the electrolyte may be one or more solvents selected fromthe group consisting of ethylene carbonate, propylene carbonate,1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate,2,3-pentylene carbonate, vinylene carbonate, dimethyl carbonate, diethylcarbonate, di(2,2,2-trifluoroethyl) carbonate, dipropyl carbonate,dibutyl carbonate, ethylmethyl carbonate, 2,2,2-trifluoroethylmethylcarbonate, methylpropyl carbonate, ethylpropyl carbonate,2,2,2-trifluoroethyl propyl carbonate, methyl formate, ethyl formate,propyl formate, butyl formate, dimethyl ether, diethyl ether, dipropylether, methylethyl ether, methylpropyl ether, ethylpropyl ether, methylacetate, ethyl acetate, propyl acetate, butyl acetate, methylpropionate, ethyl propionate, propyl propionate, butyl propionate,methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate,γ-butyrolactone, 2-methyl-γ-butyrolactone, 3-methyl-γ-butyrolactone,4-methyl-γ-butyrolactone, γ-thiobutyrolactone, γ-ethyl-γ-butyrolactone,β-methyl-γ-butyrolactone, γ-valerolactone, σ-valerolactone,γ-caprolactone, s-caprolactone, β-propiolactone, tetrahydrofuran,2-methyl tetrahydrofuran, 3-methyltetrahydrofuran, trimethyl phosphate,triethyl phosphate, tris(2-chloroethyl) phosphate,tris(2,2,2-trifluoroethyl) phosphate, tripropyl phosphate, triisoproprylphosphate, tributyl phosphate, trihexyl phosphate, triphenyl phosphate,tritolyl phosphate, methyl ethylene phosphate, ethyl ethylene phosphate,dimethyl sulfone, ethyl methyl sulfone, methyl trifluoromethyl sulfone,ethyl trifluoromethyl sulfone, methyl pentafluoroethyl sulfone, ethylpentafluoroethyl sulfone, di(trifluoromethyl)sulfone,di(pentafluoroethyl) sulfone, trifluoromethyl pentafluoroethyl sulfone,trifluoromethyl nonafluorobutyl sulfone, pentafluoroethylnonafluorobutyl sulfone, sulfolane, 3-methylsulfolane,2-methylsulfolane, 3-ethylsulfolane, and 2-ethylsulfolane.

Hereinafter, the present disclosure is described in more detail on thebasis of Examples and Comparative Examples. However, Examples below areintended to illustrate the most preferred examples of the presentdisclosure, and the present disclosure is not limited to the followingExamples and Comparative Examples below.

Physical properties of samples manufactured in Examples and ComparativeExamples were measured as below, and respective batteries weremanufactured as follows.

(Discharge Output)

Output characteristics of the batteries manufactured in Examples andComparative Examples were measured in the manner of HPPC (hybrid pulsepower characterization by freedom car battery test manual.

(Low-Temperature Characteristic)

Low-temperature characteristics of the batteries manufactured in theExamples and Comparative Examples were measured through comparison incapacity with charging at 0.5C and discharging at 0.5C at roomtemperature (25° C.) and at −20° C. relative to room temperature.

(Anode)

As an anode, an active material of the carbon small surface areastructure coated on a copper substrate was used.

(Electrolyte)

1M LiPF6 solution was prepared by using a solvent containing ethylenecarbonate (EC)/ethylmethyl carbonate (EMC)/diethylcarbonate (DEC) mixedat a volume ratio of 25/45/30, and 1 wt % of vinylene carbonate (VC),0.5 wt % of 1,3-propenesultone (PRS), and 0.5 wt % of bis(oxalato)borate(LiBOB) were added thereto.

(Battery)

A cell was configured by notching a cathode plate and an anode plate,respectively, stacking the plates, and interposing a separator(polyethylene, thickness of 25 jm) between the cathode plate and theanode plate. Then, welding was performed on each tab part of the cathodeand the anode.

Next, the welded assembly of cathode/separator/anode was put into apouch, and three sides except for a liquid injection side of theelectrolyte were sealed. Here, the parts with the tab were included inthe sealing part, and the electrolyte was injected into the other part,and the liquid injection side was sealed and impregnated for 12 hours.

Then, pre-charging was performed with a current of 0.25C (2.5A) for 36minutes. After the pre-charging was completed, gas was removed and agingwas performed for 24 hours. Then, formation charge-discharge wasperformed under the charging condition with CC-CV 0.2C 4.2V 0.05CCut-off and discharging condition with CC 0.5C 2.5V Cut-off, followed bystandard charge-discharge (charging condition with CC-CV 0.5C 4.2VCut-off and discharging condition with CC 0.5C 2.5V Cut-off).

Examples 1 to 4

As the cathode active material, a lithium-transition metal oxide (CAM 1)having a concentration difference from the central part to the surfacepart was used, wherein a total composition wasLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂, the first central part had a compositionof LiNi_(0.83)Co0.1Mn_(0.07)O₂ (in Table 1, positions: 1 to 4, an errorrange of a molar ratio: ±0.01 molar ratio), the second central part hada composition of LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ (in Table 1, positions: 1to 5, an error range of a molar ratio: ±0.01 molar ratio), the surfacepart had a composition of LiNi_(0.78)Co_(0.1)Mn_(0.12)O₂ (in

Table 1, positions: 6 to 13, an error range of a molar ratio: ±0.01molar ratio). Here, the cathode active material had composition of|X3-X4|=|Z3-Z4|=0.03, |X4-X5|=Z4-Z5|=0.02, |X3-X5|=|Z3-Z5|=0.05,L₂/L=0.31, and L₃/L=0.08.

Next, as the conductive material, a mixture obtained by mixing thecarbon small surface area structure, i.e., DENKA BLACK™ (conductivematerial 1, manufacturer: Denka, Japan, product name: DENKA BLACK™) andthe carbon large surface area structure, i.e., carbon nanotube(conductive material 2) according to the composition of Table 3 belowwas used. As the binder, polyvinylidene fluoride (PVDF) having a weightaverage molecular weight of 750,000 was used.

The cathode was manufactured by using the cathode active material, theconductive material, and the binder at a weight ratio of 92:5:3,followed by coating, drying, and pressing on an aluminum substrate.Here, concentration gradients of the transition metals used in thecathode active material were shown in Table 1 below, and positions atwhich the concentrations were measured were the same as shown in FIG. 1.The concentrations were measured at an interval of 0.4 μm from thecenter of the cathode active material with regard to alithium-transition metal oxide particle having a radius of 4.8 μm.

TABLE 1 Position Ni (wt %) Co (wt %) Mn (wt %)  1 0.830 0.100 0.070  20.831 0.101 0.068  3 0.829 0.100 0.071  4 0.830 0.100 0.070  5 0.8000.099 0.101  6 0.780 0.100 0.120  7 0.780 0.100 0.120  8 0.780 0.1010.119  9 0.781 0.100 0.119 10 0.779 0.101 0.120 11 0.780 0.100 0.120 120.781 0.099 0.120 13 0.780 0.100 0.120

Examples 5 to 8

As the cathode active material, a lithium-transition metal oxide (CAM 2)having a concentration difference from the central part to the surfacepart was used, wherein a total composition wasLiNi_(0.78)Co_(0.11)Mn_(0.09)O₂ , the central part had a composition ofLiNi_(0.802)Co_(0.11)Mn_(0.888)O₂ (in Table 1, positions: 1 to 12, anerror range of a molar ratio: ±0.02 molar ratio), and the surface parthad a composition of LiNi_(0.77)Co_(0.11)Mn0.12O₂ (in Table 1, position:13, an error range of a molar ratio: ±0.01 molar ratio). Here, thecathode active material had composition of |X1-X2|=|Z1-Z2|=0.032, andL₁/L=0.92.

Next, as the conductive material, a mixture obtained by mixing thecarbon small surface area structure, i.e., DENKA BLACK™ (conductivematerial 1, manufacturer: Denka, Japan, product name: DENKA BLACK™) andthe carbon large surface area structure, i.e., carbon nanotube(conductive material 2) according to the composition of Table 3 belowwas used. As the binder, polyvinylidene fluoride (PVDF) having a weightaverage molecular weight of 750,000 was used.

The cathode was manufactured by using the cathode active material, theconductive material, and the binder at a weight ratio of 92:5:3,followed by coating, drying, and pressing on an aluminum substrate.Here, concentration gradients of the transition metals used in thecathode active material were shown in Table 2 below, and positions atwhich the concentrations were measured were the same as shown in FIG. 1.The concentrations were measured at an interval of 0.4 μm from thecenter of the cathode active material with regard to alithium-transition metal oxide particle having a radius of 4.8 μm.

TABLE 2 Position Ni (wt %) Co (wt %) Mn (wt %)  1 0.802 0.110 0.088  20.801 0.111 0.088  3 0.802 0.110 0.088  4 0.802 0.110 0.088  5 0.8030.111 0.086  6 0.802 0.110 0.088  7 0.802 0.110 0.088  8 0.802 0.1090.089  9 0.801 0.110 0.089 10 0.802 0.110 0.088 11 0.802 0.108 0.090 120.800 0.110 0.090 13 0.770 0.110 0.120

Comparative Examples 1 and 2

The cathode was manufactured in the same manner as Example 1 except thatthe content of the conductive material was changed as shown in Table 3below.

Comparative Examples 3 and 4

The cathode was manufactured in the same manner as Example 5 except thatthe content of the conductive material was changed as shown in Table 3below.

Comparative Examples 5 to 10

A battery was manufactured in the same manner as Example 1 except thatLiNi0.8Co_(0.1)Mn_(0.1)O₂ (NCM811) having a uniform composition as anentire particle was used instead of the CAM1 as the cathode activematerial.

TABLE 3 Conductive Conductive material 1 material 2 Low- Small Largetemper- surface surface Dis- ature Cathode area area charge char- activestructure structure output acteristic material (wt %) (wt %) (W/kg) (%)Example 1 CAM 1 90 10 3407 82 Example 2 CAM 1 80 20 3605 84 Example 3CAM 1 60 40 3775 90 Example 4 CAM 1 40 60 3386 81 Example 5 CAM 2 90 103264 79 Example 6 CAM 2 80 20 3523 81 Example 7 CAM 2 60 40 3640 84Example 8 CAM 2 40 60 3213 77 Comparative CAM 1 95  5 3050 70 Example 1Comparative CAM 1 20 80 3100 72 Example 2 Comparative CAM 2 95  5 303067 Example 3 Comparative CAM 2 20 80 3080 69 Example 4 ComparativeNCM811 95  5 3025 63 Example 5 Comparative NCM811 90 10 3120 67 Example6 Comparative NCM811 80 20 3150 69 Example 7 Comparative NCM811 60 403200 69 Example 8 Comparative NCM811 40 60 3050 65 Example 9 ComparativeNCM811 20 80 3000 61 Example 10

As shown in Table 3, it could be appreciated that Examples 1 to 8 inwhich the concentration gradient of the transition metal was controlledhad excellent battery output and low-temperature characteristic ascompared to those of the Comparative Examples. In particular, it couldbe appreciated that Examples 1 to 4 in which the structure of thecathode active material had the structure of the first central part, thesecond central part, and the surface part had higher discharge outputand low-temperature characteristic as compared to those of Examples 5 to8 only having the central part and the surface part.

Further, it could be appreciated that the discharge output and thelow-temperature characteristic of the battery were changed depending onthe mixing ratio of the DENKA BLACK™ and the carbon nanotube included inthe conductive material. More specifically, it could be appreciated thatwhen the content of the DENKA BLACK™ in the mixture of the DENKA BLACK™and the carbon nanotube was within the predetermined range, thedischarge output and the low-temperature characteristic were improved.

On the contrary, it could be appreciated that Comparative Examples 1 to4 in which the composition ratio of the conductive material was out ofthe range of Examples had reduced discharge output and low-temperaturecharacteristic, which was because effective contact between theconductive material and the cathode active material was not achieved. Inparticular, when the tube-shaped carbon nanotube and the sphericalnanoparticle, i.e., DENKA BLACK™, were mixed at an appropriate ratio,connection between the cathode active materials was well achieved, suchthat a conductive path was easily formed. However, when the content ofthe conductive material was biased to any one side, a large number ofpores relatively occurred, resulting in a decrease in conductivity.

Further, it could be appreciated that Comparative Examples 5 to 10 usingthe cathode active material that did not satisfy Equations 1 to 11generally had reduced discharge output and low-temperaturecharacteristic as compared to those of the Examples, and it could beconfirmed that even Comparative Examples 6 to 8 having the samecomposition ratio of the conductive material as Examples in which thedischarge output and the low-temperature characteristic were the highesthad remarkably reduced discharge output (3200 w/kg or less) andlow-temperature characteristic (70% or less).

The lithium secondary battery according to the present disclosure mayinclude the cathode containing the cathode active material in which thecentral part of one or more kinds of metals configuring the cathodeactive material has a different concentration from the surface partthereof, and simultaneously two or more kinds of conductive materialsmixed at a specific range, thereby having excellent stability and highlow-temperature characteristic and high output characteristic ascompared to a conventional lithium secondary battery.

While exemplary embodiments of the present disclosure have been shownand described above, it will be apparent to those skilled in the artthat modifications and variations could be made without departing fromthe scope of the present disclosure as defined by the appended claims.

What is claimed is:
 1. A lithium secondary battery comprising: acathode, an anode, an electrolyte, and a separator, wherein the cathodeincludes a cathode active material comprising a central part representedby Chemical Formula 1 below and a surface part represented by ChemicalFormula 2 below in the order from an inside to an outside, and aconductive material comprising a carbon large surface area structure anda carbon small surface area structure, wherein the central part has aconstant concentration region in which each of M1 concentration and M3concentration is constant in a direction from the center of the cathodeactive material to the surface part, and the constant concentrationregion has a radius of equal to or greater than 0.6 μm from the centerof the cathode active material, and wherein the cathode active materialcomprises a concentration gradient region in which M1 concentrationcontinuously decreases and M3 concentration continuously increases in adirection from the end of the constant concentration region to thesurface part:LiM1_(X1)M2_(Y1)M3_(Z1)O_(w)  [Chemical Formula 1]LiM1_(X1)M2_(Y1)M3_(Z1)O_(w)  [Chemical Formula 1]LiM1_(X2)M2_(Y2)M3_(Z2)O_(w)  [Chemical Formula 2] in Chemical Formulas1 and 2, M1, M2, and M3 are selected from the group consisting of Ni,Co, Mn, Fe, Na, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al,Ga, B, and a combination thereof, 0.75≤X1≤0.9, 0.75≤X2≤0.9, 0≤Y1≤0.2,0≤Y2≤0.2, 0.01 Z1≤0.2, 0.01≤Z2≤0.2, 0≤W≤3, 0<X1+Y1+Z1≤1, 0<X2+Y2+Z2≤1,X2≤X1, Y1≤Y2, and Z1≤Z2.
 2. The lithium secondary battery of claim 1,wherein the carbon large surface area structure and the carbon smallsurface area structure are mixed at the weight ratio of 10:90 to 70:30in the conductive material.
 3. The lithium secondary battery of claim 1,wherein the carbon large surface area structure and the carbon smallsurface area structure are mixed at the weight ratio of less than10:more than 90 in the conductive material.
 4. The lithium secondarybattery of claim 1, wherein the carbon large surface area structure andthe carbon small surface area structure are mixed at the weight ratio ofmore than 70:less than 30 in the conductive material.
 5. The lithiumsecondary battery of claim 1, wherein the conductive material furthercomprises any one or two or more selected from a conductive fiber, afluorinated carbon, a metal powder, a conductive whisker, a conductiveoxide and a conductive polymer.
 6. The lithium secondary battery ofclaim 1, wherein the carbon large surface area structure comprises anyone or two or more selected from graphene, single-walled carbon nanotubeand multi-walled carbon nanotube, and the carbon small surface areastructure comprises any one or two or more selected from fullerene,graphite, black lead, carbon black, acetylene black, Ketjen black, Denkablack, thermal black, channel black, furnace black, lamp black, thermalblack and Super-P.
 7. The lithium secondary battery of claim 1, whereinthe cathode active material satisfies Equations 1 and 2 below:$\begin{matrix}{0.01 \leq {{{X\; 1} - {X\; 2}}} \leq 0.05} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack \\{0.01 \leq {{{Z\; 1} - {Z\; 2}}} \leq {0.05.}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$
 8. The lithium secondary battery of claim 1, wherein thecathode active material satisfies Equation 9 below: $\begin{matrix}{0.5 \leq {L_{1}/L} \leq {{0.9}9}} & \lbrack {{Equation}\mspace{14mu} 9} \rbrack\end{matrix}$ in Equation 9, L₁ is a distance from the center of thecathode active material to a boundary between the surface part and thecentral part, and L is a radius of an entire cathode active material. 9.The lithium secondary battery of claim 1, wherein the surface part has awidth of equal to or greater than 0.4 μm from the outermost surface ofthe cathode active material.